Since a plasma display panel (hereinafter, referred to as “PDP”) can realize both high definition and a large screen, 65-inch class televisions have been commercialized. Recently, PDPs have been applied to full high-definition in which the number of scan lines is twice or more than that of a conventional NTSC method. Meanwhile, from the viewpoint of environmental problems, PDPs which do not contain lead have been demanded.
A PDP basically includes a front panel and a rear panel. The front panel includes a glass substrate of sodium borosilicate glass produced by a float process; display electrodes each composed of striped transparent electrode and metal bus electrode formed on one principal surface of the glass substrate; a dielectric layer covering the display electrodes and functioning as a capacitor; and a protective layer made of magnesium oxide (MgO) formed on this dielectric layer. On the other hand, the rear panel includes a glass substrate; striped address electrodes formed on one principal surface of the glass substrate; a base dielectric layer covering the address electrodes; barrier ribs formed on the base dielectric layer; and phosphor layers formed between the barrier ribs and emitting red, green and blue light, respectively.
The front panel and the rear panel are hermetically sealed and arranged so that their surfaces having electrodes face each other. Discharge gas of Ne—Xe is filled in a discharge space participated by the barrier ribs at a pressure of 400 Torr to 600 Torr. The PDP realizes a color image display by selectively applying a video signal voltage to a display electrode so as to generate electric discharge, and exciting a phosphor layer of each color with an ultraviolet ray generated by the electric discharge so as to emit red, green and blue light.
For the metal bus electrode of the display electrode, a silver electrode for securing electric conductivity is used, and for the dielectric layer, a low melting point glass material containing lead oxide as a main component is used. Recently, however, from the viewpoint of environmental problems, examples in which a dielectric layer does not contain a lead component have been disclosed (see, for example, patent documents 1, 2 and 3).
As previously mentioned, PDPs have been applied to full high-definition in which the number of scan lines is twice or more than that of a conventional NTSC method. With such a trend toward high definition, the number of scan lines is increased, and the number of display electrodes is increased. Furthermore, the interval between the display electrodes is reduced.
Therefore, diffusion of silver ions from a silver electrode constituting a display electrode to a dielectric layer is increased. When silver ions diffuse to the dielectric layer, they are subjected to a reduction reaction with alkali metal ions in the dielectric layer. Thus, silver oxide colloid is formed. Then, this silver oxide allows the dielectric layer to be strongly colored yellow or brown. At the same time, a part of the silver oxide is subjected to a reduction reaction so as to generate oxygen bubbles, and the bubbles cause insufficient insulation.
Then, it is proposed that a low melting point glass material such as bismuth oxide, which suppresses the reaction with a silver electrode, be used for a dielectric layer instead of using a lead component. However, when a large amount of low melting point glass materials such as bismuth oxide is used for a dielectric layer, the visible light transmittance of the dielectric layer is remarkably lowered. When the amount of the low melting point glass material such as bismuth oxide is reduced in order to suppress the lowering of the visible light transmittance of the dielectric layer, the reaction with silver electrode cannot be suppressed sufficiently, which may cause coloring and insufficient insulation.
Thus, a conventional dielectric layer which does not contain a lead component, which has been proposed from the viewpoint of environmental problems, has had a problem that it is difficult to achieve both the prevention of coloring and insufficient insulation and the suppression of lowering of the visible light transmittance in the dielectric layer.
[Patent document 1] Japanese Patent Unexamined Publication No. 2003-128430
[Patent document 2] Japanese Patent Unexamined Publication No. 2002-053342
[Patent document 3] Japanese Patent Unexamined Publication No. H9-050769